I had this problem as well though with Nikon files. It appears to be the DNG, not the type of file it comes from. I solved it by going Media Pro -> Media Rendering and then under the Raw tab, I set the Rendering Engine to Phase One. That solved my problems. That being said, since I've started with LR4, I haven't added any new files to my MediaPro catalogs, prefering just to use LR as my one stop shop for DAM. Still use Media Pro for all my old files though until I spend the weeks of tedium importing all my old files (100,000+) LR4.

Hey, just wanted to let everyone know, turns out that there's a new feature in LR4 when converting to DNG. If you check the EMBED FASTLOAD DATA when converting, the resulting DNG's embedding JPEG won't be seen by OSX.

That only took 8 months to figure out. Adobe Help needs some updating...